This invention relates to a method and composition for preparing set gypsum-containing products, e.g., gypsum boards, reinforced gypsum composite boards, plasters, machinable materials, joint treatment materials, and acoustical tiles, and methods and compositions for producing them. More particularly, the invention concerns such set gypsum-containing products that have increased resistance to permanent deformation (e.g., sag resistance) by employing one or more enhancing materials. Some preferred embodiments of the invention concern making such products by hydration of calcined gypsum in the presence of an enhancing material that causes the set gypsum produced by such hydration to have increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability (e.g., non-shrinkage during drying of set gypsum). The enhancing material also provides other improved properties and advantages in preparing the set gypsum-containing products. In an alternative embodiment of the invention, set gypsum is treated with one or more enhancing materials to provide similar, if not the same, increased strength, resistance to permanent deformation (e.g., sag resistance), dimensional stability, and other improved properties and advantages in gypsum-containing products. In some embodiments of the invention, the set gypsum-containing product of the invention contains relatively high concentrations of chloride salts, yet avoids detrimental effects of such salt concentrations in gypsum-containing products in general.
Many well known useful products contain set gypsum (calcium sulfate dihydrate) as a significant, and often as the major, component. For example, set gypsum is the major component of paper-faced gypsum boards employed in typical drywall construction of interior walls and ceilings of buildings (see, e.g., U.S. Pat. Nos. 4,009,062 and 2,985,219). It is also the major component of gypsum/cellulose fiber composite boards and products, as described in U.S. Pat. No. 5,320,677. Products that fill and smooth the joints between edges of gypsum boards often contain major amounts of gypsum (see, e.g., U.S. Pat. No. 3,297,601). Acoustical tiles useful in suspended ceilings can contain significant percentages of set gypsum, as described, for example, in U.S. Pat. Nos. 5,395,438 and 3,246,063. Traditional plasters in general, e.g., for use to create plaster-surfaced internal building walls, usually depend mainly on the formation of set gypsum. Many specialty materials, such as a material useful for modeling and mold-making that can be precisely machined as described in U.S. Pat. No. 5,534,059, contain major amounts of gypsum.
Most such gypsum-containing products are prepared by forming a mixture of calcined gypsum (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water (and other components, as appropriate), casting the mixture into a desired shaped mold or onto a surface, and allowing the mixture to harden to form set (i.e., rehydrated) gypsum by reaction of the calcined gypsum with the water to form a matrix of crystalline hydrated gypsum (calcium sulfate dihydrate). This is often followed by mild heating to drive off the remaining free (unreacted) water to yield a dry product. It is the desired hydration of the calcined gypsum that enables the formation of an interlocking matrix of set gypsum crystals, thus imparting strength to the gypsum structure in the gypsum-containing product.
All of the gypsum-containing products described above could benefit if the strength of their component set gypsum crystal structures were increased in order to make them more resistant to the stresses they may encounter during use.
Also there is a continuing effort to make many such gypsum-containing products lighter in weight by substituting lower density materials (e.g., expanded perlite or air voids) for part of their set gypsum matrix In such cases there is a need to increase the strength of the set gypsum above normal levels just to maintain overall product strength at the levels of the previously higher density product, because there is less set gypsum mass to provide strength in the lower density product.
Furthermore, there is a need for greater resistance to permanent deformation (e.g., sag resistance) in the structure of many of these gypsum-containing products, especially under conditions of high humidity and temperature, or even load. The human eye typically cannot perceive sag of a gypsum-containing board at less than about 0.1 inch of sag per two foot length of board. Thus, there is a need for gypsum-containing products that are resistant to permanent deformation over the useful life of such products. For example, gypsum-containing boards and tiles are often stored or employed in a manner in which they are positioned horizontally. If the set gypsum matrix in these products is not sufficiently resistant to permanent deformation, especially under high humidity and temperature, or even load, the products may start to sag in areas between the points where they are fastened to or supported by an underlying structure. This can be unsightly and can cause difficulties in use of the products. In many applications gypsum-containing products must be able to carry loads, e.g., insulation or condensation loads, without perceptible sag. Thus, there is a continuing need to be able to form set gypsum having increased resistance to permanent deformation (e.g., sag resistance).
There is also a need for greater dimensional stability of set gypsum in gypsum-containing products during their manufacture, processing, and commercial application. Especially under conditions of changing temperature and humidity, set gypsum can shrink or expand. For example, moisture taken up in crystal interstices of a gypsum matrix of a gypsum board or tile exposed to high humidity and temperature can aggravate a sagging problem by causing the humidified board to expand. Also, in the preparation of set gypsum products there is usually a significant amount of free (unreacted) water left in the matrix after the gypsum has set. This free water is usually subsequently driven off by mild heating. As the evaporating water leaves the crystal interstices of the gypsum matrix, the matrix tends to shrink from natural forces of the set gypsum (i.e., the water was holding apart portions of the interlocking set gypsum crystals in the matrix, which then tend to move closer together as the water evaporates).
If such dimensional instability could be avoided or minimized, various benefits would result. For example, existing gypsum board production methods would yield more product if the boards did not shrink during drying, and gypsum-containing products desired to be relied on to hold a precise shape and dimensional proportions.(e.g., for use in modeling and mold making) would serve their purposes better. Also, for example, some plasters intended for interior building wall surfaces could benefit from not shrinking during drying, so that the plaster could be applied in thicker layers without danger of cracking, rather than needing to be applied in multiple thinner layers with long pauses to allow adequate drying between layer applications.
Some particular types of gypsum-containing products also exhibit other particular problems. For example, lower density gypsum-containing products are often produced by using foaming agents to create aqueous bubbles in calcined gypsum slurries (flowable aqueous mixtures) that yield corresponding permanent voids in the product when the set gypsum forms. It is often a problem that, because the aqueous foams employed are inherently unstable and therefore many of the bubbles may coalesce and escape the relatively dilute slurry (like bubbles in a bubble bath) before the set gypsum forms, significant concentrations of foaming agents have to be employed to produce the desired concentration of voids in the set gypsum, in order to obtain a product of desired density. This increases costs and risks of adverse effects of chemical foaming agents on other components or properties of the gypsum-containing products. It would be desirable to be able to reduce the amount of foaming agent needed to produce a desired void concentration in set gypsum-containing products.
There is also a need for new and improved compositions and methods for producing set gypsum-containing products made from mixtures containing high concentrations (i.e., at least 0.015 weight percent, based on the weight of calcium sulfate materials in the mixture) of chloride ions or salts thereof. The chloride ions or salts thereof may be impurities in the calcium sulfate material itself or the water (e.g., sea water or brine-containing subsurface water) employed in the mixture, which prior to the present invention could not be used to make stable set gypsum-containing products.
There is also a need for new and improved compositions and methods for treating set gypsum to improve strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability.
Thus, there is a continuing need for new and improved set gypsum-containing products, and compositions and methods for producing them, that solve, avoid, or minimize the problems noted above. The present invention meets these needs.
The present inventors have unexpectedly found set gypsum-containing products and compositions and methods for their preparation that unexpectedly meet the needs described above. Each embodiment of the invention meets one or more of these needs.
A set gypsum-containing product of the invention having increased resistance to permanent deformation is prepared in accordance with the invention by forming a mixture of a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from: condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units.
The mixture is then maintained under conditions sufficient for the calcium sulfate material to form the improved set gypsum material.
As used herein, the term, xe2x80x9ccalcium sulfate materialxe2x80x9d, is intended to mean calcium sulfate anhydrite; calcium sulfate hemihydrate; calcium sulfate dihydrate; ions of calcium and sulfate; or mixtures of any or all thereof.
In some embodiments of the invention the calcium sulfate material is mostly calcium sulfate hemihydrate. In such cases all of the enhancing materials described above will impart increased resistance to permanent deformation to the set gypsum formed. However, some enhancing materials (e.g., the following salts, or the anionic portions thereof: sodium trimetaphosphate (also referred to herein as STMP), sodium hexametaphosphate having 6-27 repeating phosphate units (also referred to herein as SHMP), and ammonium polyphosphate having 1000-3000 repeating phosphate units (also referred to herein as APP)) will provide preferred benefits, such as greater increase in sag resistance. Also, APP provides equal sag resistance to that provided by STMP, even when added in only one fourth the STMP concentration.
In some preferred embodiments of the present invention, this is accomplished by adding trimetaphosphate ion to a mixture of calcined gypsum and water to be used to produce set gypsum-containing products (as used herein, the term, xe2x80x9ccalcined gypsumxe2x80x9d, is intended to mean alpha calcium sulfate hemihydrate, beta calcium sulfate hemihydrate, water-soluble calcium sulfate anhydrite, or mixtures of any or all thereof, and the terms, xe2x80x9cset gypsumxe2x80x9d and xe2x80x9chydrated gypsumxe2x80x9d, are intended to mean calcium sulfate dihydrate). When the water in the mixture reacts spontaneously with the calcined gypsum to form set gypsum, the set gypsum is unexpectedly found to have increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability, compared with set gypsum formed from a mixture containing no trimetaphosphate ion. The mechanism for these improvements in properties is not understood.
Furthermore, it has been unexpectedly found that trimetaphosphate ion (like APP) does not retard the rate of the formation of set gypsum from calcined gypsum. In fact, when added at relatively higher concentration levels within its useful ranges of addition, trimetaphosphate ion actually accelerates the rate of hydration of calcined gypsum to form set gypsum. This is especially surprising, as is the increase in the strength of the set gypsum, because it has been generally thought in the gypsum art that phosphoric or phosphate materials retard the rate of formation of set gypsum and decrease the strength of the gypsum formed. This is in fact true for most such materials, but not for trimetaphosphate ion.
Thus, in general, some preferred embodiments of the invention provide a method for producing a set gypsum-containing product having increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability, comprising: forming a mixture of calcined gypsum, water, and trimetaphosphate ion, and maintaining the mixture under conditions (e.g., a temperature preferably less than about 120F) sufficient for the calcined gypsum to convert to set gypsum.
In some preferred embodiments of the invention the method is one of producing a gypsum board comprising a core of set gypsum sandwiched between cover sheets of paper or other material. The board is prepared by forming a flowable mixture (slurry) of calcined gypsum, water, and trimetaphosphate ion, depositing it between cover sheets, and allowing the resultant assembly to set and dry.
While the board thus produced has all of the desired improved properties of increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability, it has been observed that, for reasons unknown, when such a board has for some reason become wet or has not been completely dried during production, the bond between the gypsum core and the cover sheets (usually comprising paper) can lose strength or even fail, even when the board contains a typical nonpregelatinized starch (e.g., an acid-modified starch) which normally contributes to better paper-to-core bond integrity. The cover sheets could then delaminate from the board, which would be unacceptable. Fortunately the present inventors have also found a solution to this possible attendant problem. They have found that the problem can be avoided by including a pregelatinized starch in the production slurry. This starch then becomes distributed throughout the resultant gypsum core, and it has been unexpectedly found that this avoids the weakening of the bonding between the core and the cover sheets.
Thus, in some of its embodiments the invention provides a composition and method for producing an even more improved gypsum board. The composition comprises a mixture of water, calcined gypsum, trimetaphosphate ion, and a pregelatinized starch. The method comprises forming such a mixture, depositing it between cover sheets and allowing the resultant assembly to set and dry.
In cases where it is desired to produce a gypsum board of lighter weight, the invention provides a composition and method for accomplishing this. The composition comprises a mixture of water, calcined gypsum, trimetaphosphate ion, and an aqueous foam, and the method comprises forming such a mixture, depositing it between cover sheets, and allowing the resultant assembly to set and dry. Such composition and method provide a board of lighter weight, because the bubbles of aqueous foam result in corresponding air voids in the set gypsum core of the resultant board. The overall strength of the board is higher than a prior art board produced with the inclusion of an aqueous foam in the mixture, because of the increased strength provided by the inclusion of the trimetaphosphate ion in the mixture used to form the inventive board. For example, ceiling boards of xc2xd inch thickness made in accordance with the present invention have greater resistance to permanent deformation (e.g., sag resistance) than ⅝ inch ceiling boards made using prior art compositions and methods. Thus, the present invention provides substantial cost savings for ceiling board production.
Unexpectedly, there has been found to be another benefit to the inclusion of trimetaphosphate ion in mixtures also containing an aqueous foam. Namely, it has been found that proportionally more air voids (and more overall air void volume) per unit amount of aqueous foam employed, are created in the resultant gypsum-containing product when trimetaphosphate ion is included in the mixture. The reason for this is not known, but the beneficial result is that less foaming agent has to be employed to produce the desired amount of air void volume in the set gypsum-containing product. This in turn results in lower production costs and less risk of adverse effects of chemical foaming agents on other components or properties of the gypsum-containing product.
In some embodiments the invention provides a composite board comprising set gypsum and a reinforcing material, prepared by: forming or depositing a mixture on a surface, wherein the mixture comprises the reinforcing material, a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate material to form a set gypsum material.
The invention also provides a composite board comprising set gypsum and host particles, at least a portion of the set gypsum being positioned in and about accessible voids in the host particles. The board is prepared by forming or depositing a mixture on a surface, wherein the mixture comprises: the host particles; calcium sulfate hemihydrate, at least a portion of which is in the form of crystals in and about the voids of the host particles; water; and an appropriate amount of one or more enhancing materials chosen from the group consisting of condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate hemihydrate to form set gypsum, whereby the portion of the set gypsum in and about the accessible voids in the host particles forms by in situ hydration of the calcium sulfate hemihydrate crystals in and about the voids of the host particles.
The invention also provides a set gypsum-containing machinable product prepared by forming a mixture comprising a starch, particles of a water-redispersible polymer, a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from: condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate material to form a set gypsum material.
The invention also provides a set gypsum-containing product employed to finish a joint between edges of gypsum boards, the product prepared by inserting into the joint a mixture comprising a binder, a thickener, a non-leveling agent, a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate material to form a set gypsum material.
The invention also provides a set gypsum-containing acoustical tile prepared by forming or depositing in a tray a mixture comprising a gelatinized starch, a mineral wool, a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate material to form a set gypsum material.
The invention also provides another type of set gypsum-containing acoustical tile prepared by forming or depositing in a tray a mixture comprising a gelatinized starch, expanded perlite particles, a fiber reinforcing agent, a calcium sulfate material, water, and an appropriate amount of one or more enhancing materials chosen from condensed phosphoric acids, each of which comprises 2 or more phosphoric acid units; and salts or ions of condensed phosphates, each of which comprises 2 or more phosphate units. The mixture is then maintained under conditions sufficient for the calcium sulfate material to form a set gypsum material.
The invention also provides set gypsum-containing products made by forming a mixture of enhancing material, calcium sulfate dihydrate and water. More specifically, these embodiments involve the treatment of gypsum cast with enhancing material. Formation of a mixture of the enhancing material, water, and calcium sulfate dihydrate has been found to provide set gypsum-containing products having increased strength, resistance to permanent deformation (i.e., sag resistance), and dimensional stability. Such post set treatment can be accomplished by addition of the enhancing material by either spraying or soaking the calcium sulfate dihydrate cast with the enhancing material.
In some embodiments the invention provides a composition and method for producing set gypsum-containing products from mixtures containing high concentrations of chloride ions or salts thereof (i.e., at least 0.015 weight percent, based on the weight of calcium sulfate materials in the mixture). The chloride ions or salts thereof may be impurities in the calcium sulfate material itself or the water (e.g., sea water or brine-containing subsurface water) employed in the mixture, which prior to the present invention could not be used to make stable set gypsum-containing products.